1. Field of the Invention
The present invention relates generally to the size reduction of materials, and, more particularly, to a system for controlling the material throughput of a size-reduction plant such as a rock crusher used in an aggregate production plant.
2. Description of the Related Art
A typical materials size reduction plant environment such as a rock crushing plant is a hazardous environment in which processing and process control equipment must be able to withstand the shocks and other hazards associated with tumbling rock and aggregate material. Moreover, the processing system must be capable of accommodating changes in operating parameters that become necessary in accordance with changing needs.
For example, during a typical processing run, variations in coarseness, hardness, size and moisture content typically occur. These variations may be due to variations in the large-size material as it enters a crusher bowl, or may be due to factors that develop within the material as it sits in the crusher bowl waiting to be crushed. Such variations can affect the speed at which the equipment crushes the rock, as well as the speed at which the rock itself physically moves through the system.
Previously, the art has attempted to regulate the throughput of material by incorporating contact-type or noncontact-type means for sensing the level of material accumulating within the crusher bowl, the notion being that knowledge of the rising or falling level of material within the crusher bowl can be translated into adjustment of the flow rate of material entering the crusher bowl.
One type of sensor known to the art is disclosed in the applicant's prior U.S. Pat. No. 4,804,148. The crusher control system disclosed in that patent includes a suspended sensor, including a probe assembly that is contacted by material rising within the crusher bowl. When the material reaches a certain level, it contacts the probe, causing the probe to trip a microswitch, which in turn sends a signal to external circuitry for diverting a portion of the incoming material away from the crusher bowl until the level of material within the crusher bowl subsides.
Other attempted solutions include noncontact-type solutions such as the ultrasonic level indicator disclosed in U.S. Pat. No. 4,909,449 to Etheridge. This solution works on certain crushers, but is relatively expensive, and requires specially trained engineers for setup and maintenance. Moreover, the ultrasonic level indicator cannot be employed with certain types of existing crushers, particularly those having distributor plates that move and thus limit the area of detection.